1. Field of the Invention
The present invention relates to an optical apparatus and, more particularly, to a data recording/reproducing apparatus for recording data on or reproducing data from an optical recording medium.
2. Description of the Related Art
As one type of data recording/reproducing apparatus, optical disk apparatus have been developed. A conventional optical disk apparatus records data in the form of pits along a spiral track of an optical disk by means of an optical head and reproduces the data from the pits recorded on the disk. The area between the pits is called a land portion. The optical head comprises a light source for directing a light beam to the optical disk, an objective lens and a plurality of photoelectric conversion elements for detecting reflected light from the optical disk. An output current for each of the conversion elements detected by a corresponding photoelectric element is converted to a voltage by a corresponding current-to-voltage converter. Output voltages of the current-to-voltage converters are subjected to arithmetic operations, such as addition and subtraction, to produce a focus signal, a tracking signal and a reproduced signal for recorded data. The focus signal is used for focus control which keeps the distance between the disk and the objective lens constant to correct a focus error resulting from vertical movement of the surface of the disk. The tracking signal is used for radial tracking which controls the light beam not to deviate from a track during recording or reproducing. The focus signal and tracking signal cover a frequency range of DC--20 KHz, while the reproduced signal for recorded data covers a wide range of frequencies of DC--several MHz.
U.S. Pat. No. 4,701,897 discloses such an optical disk apparatus.
The conventional optical disk apparatus described above has the following drawbacks.
First, irrespective of the focus control, a focus error might be caused because of an external shock or the like. Conventionally, to detect this type of focus error, a special sensor is used or an average value of output currents of the photoelectric elements is calculated. The average value of the output currents is lowered when the focus error occurs. When the special sensor is used, however, the number of parts used increases, complicating the construction of the disk device. During reproduction the average amount of the reflected light from pits of the optical disk also lowers as is the case with the focus error. Hence, the average value of the output currents of the photoelectric elements will make it difficult to discriminate between the recorded data and the focus error. As a result, the focus error cannot be detected accurately and thus the reproduction cannot be performed accurately.
To produce the focus control signal, the tracking signal and the reproduced signal, operational amplifiers are usually used as current-to-voltage converters. The use of a plurality of relatively wideband operational amplifiers would raise the total cost of the disk device.
A semiconductor laser is used as the light source of the optical head. The output level of the semiconductor laser during recording is set higher than that during reproducing. Thus, at a time of recording, a photodiode, which receives reflected light from the optical disk, will produce a larger output current. Where the output current is applied to a division circuit for normalization, therefore, the division circuit is required to have a greater dynamic range in order to perform an accurate arithmetic operation. As a result, the cost of parts will be raised.
For recording or reproducing, a desired track must be accessed by moving the light beam. To determine a distance traveled by the light beam, a track counting circuit is conventionally used to count the number of tracks which are crossed by the light beam. A narrowband tracking signal is used for counting the tracks. When the light beam crosses a track at high speed, therefore, the amplitude of the tracking signal becomes small, failing to obtain the correct count. As a result, the access operation becomes inaccurate and thus the accurate recording or reproduction cannot be attained.
In view of the foregoing an optical apparatus is desired which is low in the cost of parts and can record or reproduce data accurately.